The Phoenix is very similar to the other Ultimate Summonables such as the Dread Lord's Infernal or Night Elf Warden's Avatar of
Vengeance. The Phoenix is not considered a summoned unit. It burns with such intensity that it damages itself over time.
The Phoenix damages itself at 25 hit points per second.
The Phoenix can fly unlike those other examples.

Phoenixes shed feathers, which cause both initial damage as well as a certain amount of damage over time to nearby hostile
units.

When a Phoenix dies, it becomes a Phoenix Egg. If the Phoenix Egg is not destroyed within 10 seconds, the Phoenix will be reborn.